[feature] Export cppstd_flag as public tool (#15710)

* Use ConanFile for cppstd_flag

Signed-off-by: Uilian Ries <uilianries@gmail.com>

* expose cppstd_flag

Signed-off-by: Uilian Ries <uilianries@gmail.com>

* lazy import to break cyclic import

Signed-off-by: Uilian Ries <uilianries@gmail.com>

* centralize to_cppstd_flag

Signed-off-by: Uilian Ries <uilianries@gmail.com>

* Update method description

Signed-off-by: Uilian Ries <uilianries@gmail.com>

* improve method documentation

Signed-off-by: Uilian Ries <uilianries@gmail.com>

* Revert "centralize to_cppstd_flag"

This reverts commit a98b289.

---------

Signed-off-by: Uilian Ries <uilianries@gmail.com>

conan/tools/apple/xcodetoolchain.py

@@ -57,7 +57,7 @@ def generate(self):
     @property
     def _cppstd(self):
         from conan.tools.build.flags import cppstd_flag
-        cppstd = cppstd_flag(self._conanfile.settings)
+        cppstd = cppstd_flag(self._conanfile)
         if cppstd.startswith("-std="):
             return cppstd[5:]
         return cppstd

conan/tools/build/__init__.py

@@ -3,6 +3,7 @@
 import sys
 from shlex import quote
 
+from conan.tools.build.flags import cppstd_flag
 from conan.tools.build.cppstd import check_max_cppstd, check_min_cppstd, \
     valid_max_cppstd, valid_min_cppstd, default_cppstd, supported_cppstd
 from conan.tools.build.cpu import build_jobs

conan/tools/build/flags.py

@@ -1,4 +1,3 @@
-from conan.tools.apple.apple import _to_apple_arch
 from conans.model.version import Version
 
 
@@ -7,6 +6,7 @@ def architecture_flag(settings):
     returns flags specific to the target architecture and compiler
     Used by CMakeToolchain and AutotoolsToolchain
     """
+    from conan.tools.apple.apple import _to_apple_arch
     compiler = settings.get_safe("compiler")
     arch = settings.get_safe("arch")
     the_os = settings.get_safe("os")
@@ -165,10 +165,23 @@ def build_type_flags(settings):
     return []
 
 
-def cppstd_flag(settings):
-    compiler = settings.get_safe("compiler")
-    compiler_version = settings.get_safe("compiler.version")
-    cppstd = settings.get_safe("compiler.cppstd")
+def cppstd_flag(conanfile) -> str:
+    """
+    Returns flags specific to the C++ standard based on the ``conanfile.settings.compiler``,
+    ``conanfile.settings.compiler.version`` and ``conanfile.settings.compiler.cppstd``.
+
+    It also considers when using GNU extension in ``settings.compiler.cppstd``, reflecting it in the
+    compiler flag. Currently, it supports GCC, Clang, AppleClang, MSVC, Intel, MCST-LCC.
+
+    In case there is no ``settings.compiler`` or ``settings.cppstd`` in the profile, the result will
+    be an **empty string**.
+
+    :param conanfile: The current recipe object. Always use ``self``.
+    :return: ``str`` with the standard C++ flag used by the compiler. e.g. "-std=c++11", "/std:c++latest"
+    """
+    compiler = conanfile.settings.get_safe("compiler")
+    compiler_version = conanfile.settings.get_safe("compiler.version")
+    cppstd = conanfile.settings.get_safe("compiler.cppstd")
 
     if not compiler or not compiler_version or not cppstd:
         return ""

conan/tools/gnu/autotoolstoolchain.py

@@ -44,7 +44,7 @@ def __init__(self, conanfile, namespace=None, prefix="/"):
         self.build_type_flags = build_type_flags(self._conanfile.settings)
         self.build_type_link_flags = build_type_link_flags(self._conanfile.settings)
 
-        self.cppstd = cppstd_flag(self._conanfile.settings)
+        self.cppstd = cppstd_flag(self._conanfile)
         self.arch_flag = architecture_flag(self._conanfile.settings)
         self.libcxx, self.gcc_cxx11_abi = libcxx_flags(self._conanfile)
         self.fpic = self._conanfile.options.get_safe("fPIC")

conan/tools/google/toolchain.py

@@ -76,7 +76,7 @@ def __init__(self, conanfile):
         #: String used to add --dynamic_mode=["fully"|"off"]. Depends on self.options.shared value.
         self.dynamic_mode = "fully" if shared else "off"
         #: String used to add --cppstd=[FLAG]. Depends on your settings.
-        self.cppstd = cppstd_flag(self._conanfile.settings)
+        self.cppstd = cppstd_flag(self._conanfile)
         #: List of flags used to add --copt=flag1 ... --copt=flagN
         self.copt = []
         #: List of flags used to add --conlyopt=flag1 ... --conlyopt=flagN

conan/tools/microsoft/nmaketoolchain.py

@@ -54,7 +54,7 @@ def _cl(self):
         cflags.extend(self.extra_cflags)
 
         cxxflags = []
-        cppstd = cppstd_flag(self._conanfile.settings)
+        cppstd = cppstd_flag(self._conanfile)
         if cppstd:
             cxxflags.append(cppstd)
         cxxflags.extend(self._conanfile.conf.get("tools.build:cxxflags", default=[], check_type=list))

conans/test/unittests/client/build/cpp_std_flags_test.py

@@ -1,16 +1,17 @@
 import unittest
 
 from conan.internal.api.detect_api import default_cppstd
-from conan.tools.build.flags import cppstd_flag
+from conan.tools.build import cppstd_flag
 from conans.model.version import Version
-from conans.test.utils.mocks import MockSettings
+from conans.test.utils.mocks import MockSettings, ConanFileMock
 
 
 def _make_cppstd_flag(compiler, compiler_version, cppstd=None):
-    settings = MockSettings({"compiler": compiler,
+    conanfile = ConanFileMock()
+    conanfile.settings = MockSettings({"compiler": compiler,
                              "compiler.version": compiler_version,
                              "compiler.cppstd": cppstd})
-    return cppstd_flag(settings)
+    return cppstd_flag(conanfile)
 
 
 def _make_cppstd_default(compiler, compiler_version):

conans/test/unittests/tools/intel/test_intel_cc.py

@@ -46,15 +46,16 @@ def test_architecture_flag_if_intel_cc(os_, arch, expected):
     ("gnu23", "gnu++2b"),
 ])
 def test_cppstd_flag_if_intel_cc(cppstd, flag):
-    settings = MockSettings({
+    conanfile = ConanFileMock()
+    conanfile.settings = MockSettings({
         "compiler": "intel-cc",
         "compiler.version": "2021.3",
         "compiler.mode": "classic",
         "arch": "x86_64",
         "os": "Linux",
         "compiler.cppstd": cppstd
     })
-    assert cppstd_flag(settings) == "-std=%s" % flag
+    assert cppstd_flag(conanfile) == "-std=%s" % flag
 
 
 @pytest.mark.parametrize("mode", ["icx", "dpcpp"])